Components of electronic or opto-electronic systems, such as for example Integrated Circuits (ICs) like data and/or signal processors (microprocessors, CPUs, DSPs, ASICs), photodetectors, semiconductor lasers, are usually packaged in respective packages, and then assembled to boards (Printed Circuit Boards—PCBs) that perform a double function of providing a mechanical support as well as a functional (electrical) interconnection between the different systems' components.
One packaging solution known in the art, featuring a high packing efficiency, is the so-called Multi-Chip-Module (MCM) packaging: in an MCM package, two or more IC chips (dies) are directly attached, typically soldered (for example, using so-called C4—Controlled Collapse Chip Connections—technique) to a common substrate, the MCM substrate, e.g. of organic material; the MCM package, rather than the individual IC chips, is then assembled to the PCB.
Electronic components' packages has the primary function of protecting the packaged components mechanically and from attacks by agents in the external environment.
However, in applications involving high operating speeds, which are becoming more and more common, a further requirement of the packages is that they essentially maintain the performance levels of the electronic components they carry within, or that they affect the packaged components' performances as less as possible. An example of high-speed application where the characteristics of components' packages are critical are opto-electronic systems, wherein high switching speed electrical signals often have to be converted into optical signals, and vice versa.
In particular, when the signals' switching speed exceeds the Gigahertz, a proper analysis of the electronic system needs to take into account the wave nature of the electromagnetic field: the transmission of an electric signal (e.g., a voltage) needs to be considered from the viewpoint of an electromagnetic wave that propagates through the circuit, being supported by an electric current in a circuit's conductive trace.
The solutions adopted for packaging the electronic components of a high-speed system affect the propagation of the electromagnetic wave.
An important role is played by the properties of the packages' materials: it is for example known that the materials' dielectric constants and dielectric losses affect the electromagnetic wave propagation. Another aspect that impacts the performance of the packaged components is the package structure (such as its spatial configuration).
As a result, a package, if not carefully designed and selected, may have such an impact on the packaged component's performance (e.g., the package may affect the propagation of electromagnetic waves corresponding to the signals generated or received by the packaged component to such an extent) that the packaged component becomes almost inoperable, at the expected operating speed.
For example, considering the case of an opto-electronic system, light-emitting devices (e.g., laser diodes) used to convert electrical signals into optical signals need to receive electrical signals already modulated at high speed, generated for example by a microprocessor: a bad electrical signal transmission from the signal generator to the electro-optical converter translates into a bad optically converted signal. Similar considerations apply to the reverse signal conversion, from optical into electrical: the high-speed electrical signals generated by, e.g., a photodetector, like a photodiode, must not be worsened too much in the propagation from the photodetector to the IC(s) that have to process the converted electrical signals.
Packages for components of electronic systems thus need to be designed in such a way that they do not affect, as far as possible, the propagation of electromagnetic waves associated with the electric signals generated/received by the packaged ICs.
To this purpose, a known countermeasure calls for designing and realizing circuit structures having a carefully controlled impedance value across a generic signal transmission line.
Controlling the transmission line impedance value is however not sufficient, due to the unavoidable presence of parasitic elements exhibiting a capacitive, resistive or inductive behavior, which parasitic elements are intrinsically embedded in the package, or in the PCBs, due to the association of materials and conductive structures needed to establish paths for electrical currents.
A careful selection of materials with physical properties favorable to the electromagnetic wave propagation, such as for example PTFE (PolyTetraFluoroEthylene), is not sufficient to compensate and overcome all the other effects, inherent to the package structure, e.g. the spatial configuration of MCM structures.
In particular, the propagation of electromagnetic waves is severely affected by any kind of physical discontinuity along the wave propagation path; by physical discontinuity there is intended any more or less abrupt change or transition in properties such as structure, material properties, design features.
For example, let the case be considered of an electronic system wherein electric signals for driving an electro-optical component, like a laser diode, particularly a VCSEL (Vertical Cavity Surface Emitting Laser), are generated by an IC, e.g. a CPU, which is packaged in an MCM package, and assembled to a PCB to which the VCSEL is also mounted. Several discontinuities can be observed in the signal path from the signals generator IC to the laser diode, namely the transitions from the IC signal line to the corresponding IC pad, from the IC pad to the (e.g., C4) solder bump, from the solder bump to the corresponding (e.g., C4) contact pad on the MCM's substrate, then to the conductor signal line on the MCM's substrate, from the MCM's conductor signal line to the MCM's bondage pad (e.g., a Ball Grid Array—BGA—pad) used for bonding the MCM substrate to the PCB (this transition may in particular be made up of several different transitions, corresponding for example to one or more laser vias and PTHs—Pin Through Holes), from the MCM's bondage pad to the (e.g., BGA) solder bump and to the corresponding (e.g., BGA) contact area on the PCB, then to the signal line trace on the PCB up to the laser diode. Some of the above transitions have an inherent impedance mismatch.
Experiments have demonstrated that the transition corresponding to the BGA-type bondage of the MCM substrate to the PCB has the biggest impact at high operating frequencies, having an essentially capacitive effect, and thus acting as a low-pass filter that significantly reduces the transmission line bandwidth. A lower importance, but not negligible role is played by plated through-holes in the MCM's substrate or in the PCB, C4 pads for bonding the IC chips to the MCM's substrate, laser vias and coupling effects between the signal lines and voltage supply planes.